JP2010162437A - Detoxification apparatus and detoxification method of fluorine-containing waste liquid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an detoxification apparatus and an detoxification method of a fluorine-containing waste liquid which can effectively remove an fluorine ion from the waste liquid of an alkaline scrubber. <P>SOLUTION: A NaF-containing waste liquid contained in the alkaline scrubber is stored in a water-quality regulating tank 1 and the stored waste liquid is added with HCl to be acidic. The acidic waste liquid is conveyed to a reaction tower 5 filled with CaCO<SB>3</SB>while being compressed with a pump 4. CaCO<SB>3</SB>filled in the reaction tower 5 reacts with NaF in the waste liquid to generate CaF<SB>2</SB>and Na<SB>2</SB>CO<SB>3</SB>. Since Na<SB>2</SB>CO<SB>3</SB>generated in the reaction is decomposed and removed by HCl added beforehand, a back reaction is suppressed and more fluorine ions are removed in the reaction tower 5. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an apparatus and method for removing fluorine-containing waste liquid, and more particularly to an apparatus and method for removing fluorine-containing waste liquid generated in a semiconductor manufacturing process and the like.

  In semiconductor manufacturing, a hydrogen fluoride aqueous solution (hydrofluoric acid) or a gas containing fluorine is used in an etching process of a silicon oxide film or the like. Since the waste liquid and exhaust gas generated in such an etching process contain fluorine (fluoride, fluorine ion, etc.), it is necessary to discharge after performing detoxification treatment such as fluorine removal.

  In a waste liquid containing hydrofluoric acid (HF), in the case of a waste liquid having a high hydrofluoric acid concentration, there is a method of treating the waste liquid through a reaction tank filled with calcium carbonate. According to this method, hydrofluoric acid and calcium carbonate react in a reaction tower, and fluorine is adsorbed and fixed in the reaction tower as calcium fluoride. On the other hand, in the case of a waste liquid having a low hydrofluoric acid concentration, a method of concentrating the hydrofluoric acid in the waste liquid with a chelate resin or the like, mixing the waste liquid enriched with this hydrofluoric acid with a high concentration waste liquid, and treating it in the above-mentioned reaction tower There is.

JP-A-6-63561 JP 11-156355 A

  On the other hand, for the treatment of exhaust gas containing a small amount of hydrogen fluoride, an alkali scrubber that spreads an alkaline aqueous solution on the exhaust gas and adsorbs hydrogen fluoride is used. In this alkali scrubber, fluorine in the exhaust gas is removed by a reaction represented by the following formula (1).

HF + NaOH → NaF + H ₂ O (1)
By the way, the waste liquid containing sodium fluoride generated in the above-mentioned alkali scrubber cannot be discharged as it is into a river or the like according to the environmental standard that defines the discharge standard of fluorine ions. For this reason, it is necessary to remove the fluorine ions in the waste liquid.

  Therefore, it is conceivable to treat the waste liquid from the aforementioned alkali scrubber with calcium carbonate to remove fluorine ions in the waste liquid as insoluble calcium fluoride. However, the reverse reaction is likely to occur in the reaction between sodium fluoride and calcium carbonate contained in the waste liquid from the alkali scrubber, and it is difficult to efficiently remove fluorine ions from the waste liquid.

  An object of the present invention is to provide a fluorine-containing waste liquid abatement apparatus and a detoxification method capable of efficiently removing fluorine ions from a waste liquid from an alkali scrubber.

  According to one aspect, a water quality adjusting tank for adding acid to a waste liquid containing an alkali metal fluoride salt to make the waste liquid acidic, and bringing the acidic waste liquid into contact with calcium carbonate, and fluoride ions in the waste liquid There is provided a fluorine-containing waste liquid detoxifying device comprising a reaction tower that separates from a waste liquid as calcium fluoride.

  According to the above aspect, prior to the waste liquid treatment by the reaction tower filled with calcium carbonate, acid (for example, hydrochloric acid) is added to the waste liquid containing alkali metal fluoride salt (for example, sodium fluoride) to make the waste liquid acidic. To. The sodium carbonate produced in the reaction tower is decomposed and removed by the acid added to the waste liquid. For this reason, the reaction which absorbs and fixes a fluorine ion can be accelerated | stimulated, and the removal rate of a fluorine ion improves.

FIG. 1 is a view showing a fluorine-containing waste liquid abatement apparatus according to the first embodiment. FIG. 2 is a diagram illustrating a device configuration used in the experimental example of the first embodiment. FIG. 3 is a view showing a fluorine-containing waste liquid abatement apparatus according to the second embodiment. FIG. 4 is a diagram showing the fluorine ion concentration at each stage of the fluorine-containing waste liquid abatement apparatus according to the experimental example of the second embodiment.

  Hereinafter, embodiments for carrying out the invention will be described with reference to the drawings.

(First embodiment)
Hereinafter, the detoxification method of the fluorine-containing waste liquid according to the first embodiment will be described.

  When the waste liquid containing sodium fluoride discharged from the alkali scrubber is brought into contact with calcium carbonate, the reaction shown in the following formula (2) occurs, and fluorine ions are removed as calcium fluoride.

2NaF + CaCO ₃ → CaF ₂ + Na ₂ CO ₃ (2)
The reaction shown in the above formula (2) is a substitution reaction between F ^- ions and CO ₃ ^2- ions, and proceeds reversibly. For this reason, when the concentration of sodium carbonate in the waste liquid is high, the normal reaction and the reverse reaction antagonize and the removal of sodium fluoride (fluorine ions) does not proceed. Therefore, in order to proceed with the removal of fluorine ions, it is necessary to remove sodium carbonate which is a reaction product of the formula (2).

  However, when the hydrogen ion exponent pH in the waste liquid exceeds 8.6, sodium carbonate becomes difficult to be decomposed and removed, and the removal of fluorine ions by the reaction shown in the formula (2) does not proceed.

  Since the waste liquid from the alkali scrubber has a high pH of about 10 to 11, if the waste liquid from the alkali scrubber is directly brought into contact with calcium carbonate, fluorine ions cannot be efficiently removed.

  Therefore, in this embodiment, before the waste liquid from the alkali scrubber is brought into contact with calcium carbonate, an acid is added to the waste liquid to make it acidic. In this way, when the waste liquid is acidified, sodium carbonate produced by the reaction between the waste liquid and calcium carbonate (see formula (2)) can be decomposed and removed. For example, when hydrochloric acid is added to the waste liquid, sodium carbonate is decomposed by the reaction shown in the following formula (3).

Na ₂ CO ₃ + 2HCl → H ₂ CO ₃ (dissociated into CO ₂ + H ₂ O) +2 NaCl (3)
In this way, by adding acid to the waste liquid, sodium carbonate is decomposed and removed, and the reverse reaction of formula (2) can be suppressed to remove more fluorine ions (sodium fluoride) as calcium fluoride. it can.

  In the above example, hydrochloric acid is added to the waste liquid. However, any acid other than hydrochloric acid may be used as long as it is an acid that can decompose and remove sodium carbonate.

  When the fluorine ion concentration in the waste liquid is known, the amount of hydrochloric acid added is preferably an amount capable of decomposing all sodium carbonate generated when the reaction formula (2) proceeds completely. Even when the amount of hydrochloric acid added exceeds the amount necessary for decomposing sodium carbonate, only calcium carbonate is consumed, and the fluorine ion removal performance itself does not deteriorate.

  However, if the concentration of hydrochloric acid to be added is too high, the consumption of calcium carbonate will increase. For this reason, when the fluorine ion concentration in the waste liquid exceeds 100 ppm, it is preferable to set the pH of the waste liquid to about 4 in consideration of suppression of wasteful consumption of calcium carbonate and fluorine ion removal performance.

  Further, when the pH of the waste liquid is 4.8 to 8.6, as shown by the following formula (4), sodium carbonate reacts with carbon dioxide dissolved in the waste liquid to form sodium hydrogen carbonate. Generate.

Na ₂ CO ₃ + CO ₂ (dissolved gas) + H ₂ O → 2NaHCO ₃ (4)
Depending on conditions, sodium hydrogen carbonate may cause the reverse reaction of (4) to produce sodium carbonate. In this case, the concentration of sodium carbonate in the waste liquid increases, and fluorine ion removal by the reaction of the formula (2) does not proceed. Therefore, it is preferable to add hydrochloric acid in such an amount that the pH of the waste liquid is at least 4.8 or less.

  Hereinafter, the fluorine-containing waste liquid abatement apparatus according to the first embodiment will be described. FIG. 1 is a view showing the fluorine-containing waste liquid abatement apparatus according to the first embodiment.

  As shown in FIG. 1, a detoxification apparatus 10 according to this embodiment includes a water quality adjustment tank 1 including a pH adjustment apparatus 2 and a pH measurement device 3, and a reaction connected to the water quality adjustment tank 1 via a pump 4. And a tower 5.

  The water quality adjusting tank 1 stores waste liquid containing sodium fluoride (fluorine ions) discharged from an alkali scrubber (not shown) or the like. The pH measuring device 3 measures the hydrogen ion exponent pH in the waste liquid stored in the water quality adjusting tank 1. The pH adjusting device 2 adds a predetermined amount of hydrochloric acid to the water quality adjusting tank 1 according to the measurement result of the pH measuring device 3, and the pH of the waste liquid stored in the water quality adjusting tank 1 is 4.8 or less. Preferably it is about 4.

The pump 4 sends the waste liquid stored in the water quality adjusting tank 1 to the reaction tower 5 while pressurizing it to about 1.0 to 1.2 kgf / cm ² . The pump 4 is interlocked with a liquid level sensor (not shown) provided in the water quality adjustment tank 1 and adjusts the flow rate of the waste liquid sent to the reaction tower 5 according to the position of the liquid level of the waste liquid.

  The reaction tower 5 is a pipe-shaped case filled with granular calcium carbonate having a particle size of about 0.5 mm. The reaction tower 5 is arranged vertically, and the waste liquid sent from the pump 4 is supplied from the bottom of the reaction tower 5 and flows toward the top of the reaction tower 5. The reaction tower 5 is detachably provided. By exchanging the reaction tower 5 for a certain period of time or every certain amount of waste liquid treatment, the fluorine ion removing ability of the abatement apparatus 10 can be maintained. As an example, the reaction tower 5 can be a pipe having an inner diameter of about 120 mm and filled with about 20 to 25 kg of calcium carbonate. In this case, if the waste liquid having a fluorine ion concentration of 200 ppm is treated at 2 L / min, the replacement period is about 1 to 2 months.

  In the reaction tower 5, the reaction shown in the above formula (2) proceeds, and fluorine ions (sodium fluoride) are fixed as calcium fluoride. Since sodium carbonate produced in this reaction is decomposed by hydrochloric acid added to the waste liquid, more fluorine ions can be removed according to this embodiment. The calcium fluoride accumulated in the reaction tower 5 can be recovered as a raw material of fluorine.

  Further, since the waste liquid supplied to the reaction tower 5 is pressurized by the pump 4, the generation of carbon dioxide bubbles generated by the reaction between hydrochloric acid and sodium carbonate (or calcium carbonate) is generated in the reaction tower 5. Can be suppressed. Thereby, it can prevent that the bubble of a carbon dioxide gas adheres to the surface of granular calcium carbonate, and a contact area reduces, and the fall of the fluorine ion removal capability of the reaction tower 5 can be prevented.

  As described above, before treating the waste liquid with calcium carbonate in this embodiment, hydrochloric acid is added to the waste liquid to make the waste liquid acidic. For this reason, the sodium carbonate produced | generated by reaction of sodium fluoride and calcium carbonate can be decomposed | disassembled and removed, a reverse reaction can be suppressed, and the removal efficiency of the fluorine ion by the reaction tower 5 can be improved.

(Experimental example of the first embodiment)
The inventors of the present application flowed various aqueous test solutions (samples 1 to 4) containing fluorine ions to a reaction tower filled with calcium carbonate, and examined the fluorine ion removal rate.

  FIG. 2 is a diagram showing a device configuration used in the experimental example. The reaction tower 6 of the experimental example is obtained by filling a transparent acrylic pipe having an inner diameter of 25 mm with about 500 ml of granular calcium carbonate having a particle diameter of about 0.5 mm. In the reaction tower 6, the height of the packed portion of calcium carbonate is about 1 m.

The delivery pressure of the aqueous test solution (samples 1 to 4) from the pump 4 was 1 kgf / cm ² , and the water flow rate was 50 ml / min (3 L / hr). At this time, the water flow speed of the waste liquid flowing through the reaction tower 6 is 6 m / hr.

  First, an aqueous solution (sample 1) in which a hydrogen fluoride reagent (special grade) is added to pure water in an amount such that the fluorine ion concentration is 400 ppm, and an amount in which sodium fluoride reagent (special grade) is added to pure water to have a fluorine ion concentration of 400 ppm. An aqueous solution (sample 2) was added, and the fluorine ion removal rate for these aqueous solutions was examined. The results are shown in Table 1.

表１に示すように、反応塔６によるフッ化水素中のフッ素イオンの除去率は９０％と高いのに対し、フッ化ナトリウム水溶液中のフッ素イオンの除去率は２５％程度にとどまる。フッ化水素と炭酸カルシウムとは不可逆的に反応するため、除去率が高いものと考えられる。一方、フッ化ナトリウムと炭酸カルシウムとは逆反応が生じるため、除去率が低くなってしまう。

As shown in Table 1, while the removal rate of fluorine ions in hydrogen fluoride by the reaction tower 6 is as high as 90%, the removal rate of fluorine ions in the aqueous sodium fluoride solution is only about 25%. Since hydrogen fluoride and calcium carbonate react irreversibly, it is considered that the removal rate is high. On the other hand, since the reverse reaction occurs between sodium fluoride and calcium carbonate, the removal rate becomes low.

  Therefore, sample 3 was added to pure water in an amount such that the fluorine ion concentration was 400 ppm, and hydrochloric acid was added to adjust the pH to 6.5, and sample 4 was added to hydrochloric acid to adjust the pH to 3.9. Were prepared, and the removal rate of fluorine ions with respect to these aqueous solutions was examined. The results are shown in Table 2.

表２に示すように、ｐＨ３．９となる量の塩酸を添加した試料４の場合、フッ素イオンの除去率が４０％となり、塩酸を添加しない場合（試料２）よりも除去率が向上している。これは、生成した炭酸ナトリウムを余剰に存在する塩酸で分解・除去でき、より多くのフッ素イオンを除去できることを示している。

As shown in Table 2, in the case of Sample 4 to which hydrochloric acid having an amount of pH 3.9 was added, the removal rate of fluorine ions was 40%, and the removal rate was improved compared to the case of no addition of hydrochloric acid (Sample 2). Yes. This indicates that the generated sodium carbonate can be decomposed and removed with excess hydrochloric acid, and more fluorine ions can be removed.

(Second Embodiment)
FIG. 3 is a view showing a fluorine-containing waste liquid abatement apparatus according to the second embodiment.

  As shown in FIG. 3, in the abatement apparatus 20 according to this embodiment, a plurality of reaction towers 5 are arranged in series. The reaction column 5 on the most upstream side (left side in FIG. 3) is called the first-stage reaction column 5, and the reaction column 5 arranged on the downstream side (right side in FIG. 3) is sequentially arranged in the second and third stages. ,... Shall be called the sixth stage. One water quality adjusting tank 1 is provided on the upstream side of these reaction towers 5, and a pump 4 is disposed between the water quality adjusting tank 1 and the reaction tower 5.

  The water quality adjusting tank 1 is provided with a pH measuring device 3 for measuring the pH of the waste liquid and a hydrochloric acid supply unit 2b for adding hydrochloric acid to the waste liquid stored in the water quality adjusting tank 1. The water quality adjusting tank 1 is added with an amount of hydrochloric acid according to the measurement result of the pH measuring device 3 so that the pH value of the waste liquid is 4.8 or less, preferably about 4. As a result, the waste liquid is acidified and the fluorine ion removal efficiency by the reaction tower 5 is improved. In addition, when the addition amount of hydrochloric acid necessary for setting the pH value of the waste liquid to 4.8 or less (preferably about 4) is known, the pH measuring device 3 may not be provided in the water quality adjustment tank 1. .

  Moreover, the pump 4 and the reaction tower 5 of this embodiment are the same as the pump 4 and the reaction tower 5 of 1st Embodiment. The reaction tower 5 of the second embodiment can stably remove fluorine ions only by exchanging it at regular intervals or at constant waste liquid throughputs. In this case, for example, the first-stage reaction tower 5 is removed, and the second-stage and lower reaction towers 5 are moved one by one to the upstream side, and exchanged so that a new reaction tower 5 is added on the most downstream side. Thus, stable fluorine ion removal can be performed while effectively using the reaction tower 5.

  Next, waste liquid processing by the abatement apparatus of the present embodiment will be described.

Waste liquid from the alkali scrubber is stored in the first water quality adjustment tank 1. From the hydrochloric acid supply unit 2b, an amount of hydrochloric acid is added so that the pH of the waste liquid stored in the water quality adjusting tank 1 is about 4. The waste liquid stored in the water quality adjusting tank 1 is sent to the reaction tower 5 by the pump 4 at a pressure of 1 to 1.2 kgf / cm ² .

  The waste liquid introduced into the reaction tower 5 generates calcium fluoride and sodium carbonate by the reaction of the above-described formula (2). Part of the sodium carbonate produced at this time is decomposed and removed by hydrochloric acid added in the first water quality adjustment tank 1. However, since hydrochloric acid also reacts with calcium carbonate, it is not possible to remove all of the generated sodium carbonate. For this reason, in the first-stage reaction tower 5, fluorine ions (sodium fluoride) in an amount corresponding to the sodium carbonate concentration that cannot be removed remain in the waste liquid, and this waste liquid is discharged from the first-stage reaction tower 5. Discharged.

  The waste liquid discharged from the first-stage reaction tower 5 is stored in the water quality adjusting tank 1 provided in front of the second-stage reaction tower 5. Also in this water quality adjusting tank 1, hydrochloric acid is added in such an amount that the pH of the waste liquid is 4.8 or less (more preferably about 4). Thereby, sodium carbonate contained in the waste liquid discharged from the first-stage reaction tower 5 can be removed, and sodium carbonate generated in the second-stage reaction tower 5 can be decomposed and removed.

  Thereafter, the concentration of fluorine ions in the waste liquid can be reduced stepwise by repeating the treatment by the reaction tower 5 in the second and subsequent stages and the addition of hydrochloric acid in the water quality adjusting tank 1 in the same manner.

  As described above, according to the abatement apparatus 20, the fluorine ion removal rate can be increased as compared with the case where only one reaction tower 5 is provided. In the abatement apparatus 20 of the present embodiment, the number of reaction towers 5 is not limited to six, and if necessary, the number of reaction towers 5 and water quality adjustment tanks 1 can be increased to increase fluorine. The ion removal rate can be further improved.

(Experimental example of the second embodiment)
The inventors of the present application prepared a test aqueous solution in which sodium fluoride was added to pure water in an amount such that the fluorine ion concentration became 1000 ppm, and the fluorine ion concentration when this aqueous solution was treated with the abatement apparatus 20 shown in FIG. It was measured. In addition, hydrochloric acid was added to each water quality adjustment tank 1, and pH of the stored waste liquid was set to 4.0. FIG. 4 is a diagram showing the measurement results of the concentration of fluorine ions in the aqueous solution that has passed through each reaction tower 5.

  As shown in FIG. 4, every time it passed through the reaction tower 5, fluorine ions could be removed, and about 95% of the fluorine ions could be removed by passing through the six-stage reaction tower.

DESCRIPTION OF SYMBOLS 1 ... Water quality adjustment tank, 2 ... pH adjustment apparatus, 2a ... Hydrochloric acid storage tank, 2b ... Hydrochloric acid supply part, 3 ... pH measuring device, 4 ... Pump, 5, 6 ... Reaction tower, 10, 20 ... Detoxification device.

Claims (10)

A water quality adjusting tank for acidifying the waste liquid by adding an acid to the waste liquid containing an alkali metal fluoride salt;
A detoxifying device for a fluorine-containing waste liquid, comprising: a reaction tower for bringing the acidic waste liquid into contact with calcium carbonate and separating fluoride ions in the waste liquid as calcium fluoride from the waste liquid.   2. The fluorine-containing waste liquid detoxifying apparatus according to claim 1, wherein a plurality of sets of the water quality adjusting tank and the reaction tower are connected in series.   3. The fluorine-containing waste liquid detoxifying device according to claim 1, wherein a hydrogen ion exponent pH of the waste liquid in the water quality adjusting tank is 4.8 or less.   Furthermore, the pump which sends out the waste liquid of the said water quality adjustment tank to the said reaction tower, pressurizing between the said water quality adjustment tank and the said reaction tower is provided. The fluorine-containing waste liquid abatement apparatus as described in the item.   The said acid is hydrochloric acid, The detoxification apparatus of the fluorine-containing waste liquid of any one of Claim 1 thru | or 4 characterized by the above-mentioned.   The said alkali metal fluoride salt is sodium fluoride, The detoxification apparatus of the fluorine-containing waste liquid of any one of Claim 1 thru | or 5 characterized by the above-mentioned. A water quality adjusting step for acidifying the waste liquid by adding an acid to the waste liquid containing the alkali metal fluoride salt;
A calcium carbonate treatment step in which the acidic waste liquid is contacted with calcium carbonate, and fluoride ions in the waste liquid are separated from the waste liquid as calcium fluoride;
A method for detoxifying a fluorine-containing waste liquid, comprising:   The method for removing fluorine-containing waste liquid according to claim 7, wherein the water quality adjustment step and the calcium carbonate treatment step are alternately repeated a plurality of times.   The said water quality adjustment process makes the hydrogen ion exponent pH of the said waste liquid 4.8 or less, The detoxification method of the fluorine-containing waste liquid of Claim 7 or Claim 8 characterized by the above-mentioned.   The method for removing a fluorine-containing waste liquid according to any one of claims 7 to 9, wherein the acid is hydrochloric acid.

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